This invention relates to a method and an equipment for storing holographic data, especially to a method for recording a large capacity of information with a high speed and an equipment suitable for recording a large capacity of information with a high speed.
Since an optical recording equipment and a medium have advantages such as a medium compatibility or a storage stability in comparison with the HDD (Hard Disc Drive) and faster access in comparison with a tape storage system, they have been widely used or are planned to be introduced to the equipments such as a computer back-up memory, a video recording/playback player for home-use, a navigation system for vehicle, a recorder for a video-camera, a Personal Digital Assistant, and a professional video recorder for medical, broadcasting and movie application.
In order to familiarize the optical memory equipment and expand its application, upgrading of the memory capacity and the data transfer speed is required further. The reason why the optical disc memory is used widely is that fast access capability and excellent usability are preferred.
The widely used optical disc includes a CD-ROM for a read only memory, a DVD-ROM, a WORM for write once memory, a CD-R, a DVD-R, a CD-RW for rewritable, a DVD-RAM, a DVD±RW and a MO. The above-mentioned optical disc systems use a principal that an optical beam narrowed to a diffraction limit by an objective lens is focused on the medium so as to record or read out the data. Based on this principle, there are few ways to increase the memory capacity. One is to use the shorter wavelength light. The other is to use an objective lens having a large NA (numerical aperture). Other ideas such as a mark-edge recording, a Land and Groove recording, a modulation/demodulation technique like PRML (Partial Response Maximum Likelihood), a multi-layered recording medium having a plurality of recording layers and a super resolution reading technique are proposed. However, since these methods adapt the optical system in which the recording medium surface is located in a focal point, the wavelength and the NA of the objective lens limit the memory capacity.
On the other hand, “holographic data storage” has been proposed which uses the different principal from the current optical disc recording. In this case, the focused beam narrowed to the diffraction limit is not irradiated into the recording layer. The recording layer is approximately 10,000 times thicker (for example, several hundreds microns) compared to that of the current optical disc, and recording or reading is carried out in a block of a frame or a page toward the medium thickness direction.